Marshall JCM 800 blowing fuses

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ZeroPointEnergy
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Marshall JCM 800 blowing fuses

Post by ZeroPointEnergy »

I can't keep my amp on anymore. I accidentally turned it own after moving the amp and that dislodged one of the speaker cables, and that caused a mismatch, and that caused the amp to stop working. I replaced the fuse three times now.

The first time I replaced it, the amp worked, but was quieter and soon stopped working. The second and third time, it just dies right away. I suspect my speaker cables are a problem. How do I check them?

I also blew a preamp tube, a new production Mullard. That surprised me for one of my preamp tubes to blow and so quickly. I think it was in V2 spot when it blew, not completely sure tho.

Any ideas for what to check?
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Re: Marshall JCM 800 blowing fuses

Post by JMPGuitars »

Look for anything melted or possible shorting out. Check the leads from the OT. Check you filter caps for swelling. Give it a shake and see if anything falls out.

Also check the resistance on your OT. From end to end, and from each end to CT.
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Re: Marshall JCM 800 blowing fuses

Post by Bieworm »

Is there any smell from the transformers? Had a similar problem with the blowing fuses. The cause was the sag resistor of the rectifier had a broken wire that was still touching the resistor , causing sparks and that smoked the powertranny... smelllllll
Probably not your case, but thought I'd share this anyway
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Re: Marshall JCM 800 blowing fuses

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Bieworm wrote:
Mon 06/01/20 2:54 am
Is there any smell from the transformers? Had a similar problem with the blowing fuses. The cause was the sag resistor of the rectifier had a broken wire that was still touching the resistor , causing sparks and that smoked the powertranny... smelllllll
Probably not your case, but thought I'd share this anyway
Thanks guys. So far no signs of charred parts, but the power tubes did show up as a problem, so I have a pair ordered. I'm on Marshall forums and there's a tech there walking me through bias checking and all that. Just uploaded about 9 pics of the circuit.

He's like, that amp was modded for 6550's. And it was not put fully back for your EL34's. That makes perfect sense to me, because of the amp's history. I'll explain more next time. Former amp owner liked those tubes, but I like EL34's. I guess it was a partial refurbishment job. I paid around 230 to have it done right. But I can see that's not the case.
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Re: Marshall JCM 800 blowing fuses

Post by Bieworm »

If you could get your hands on some winged =C= EL34 , then you would never worry about abuse again. They handle a LOT!!!
The bogner shiva pushes 80 watts out of 2 of them
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Re: Marshall JCM 800 blowing fuses

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He wants me to clip the white wire and then check the resistor value. After replacing some resistor somewhere, to a more proper value, that should allow my amp to be biased properly. Right now, I can't adjust it to above -35.5 V, and we were going to try 42, but can't until the bias adjustment range is taken care of.

My former amp tech nearly finished the job of converting it to EL34's, or at least, that is the impression I get. It took too long and I was loosing confidence in his ability, so he may have left it up to me about finishing it or not, but I did not realize I would need another repair session to fix what he did not finish.

Maybe this was the hardest part of the mod, and since it sounded passable to me, he avoided lifting the mainboard and possibly causing some sort of damage he did not want to fix. My amp is '82 50 Watt, JCM 800 vertical inputs, 4010 combo amp, with I believe the 2403 circuit. Not the diode clipping and transistor channel-changing version of 800, mine is the all tube signal path.

I wonder what that post is doing there..
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Re: Marshall JCM 800 blowing fuses

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ZeroPointEnergy wrote:
Sun 06/07/20 12:25 pm
He wants me to clip the white wire and then check the resistor value. After replacing some resistor somewhere, to a more proper value, that should allow my amp to be biased properly. Right now, I can't adjust it to above -35.5 V, and we were going to try 42, but can't until the bias adjustment range is taken care of.

My former amp tech nearly finished the job of converting it to EL34's, or at least, that is the impression I get. It took too long and I was loosing confidence in his ability, so he may have left it up to me about finishing it or not, but I did not realize I would need another repair session to fix what he did not finish.

Maybe this was the hardest part of the mod, and since it sounded passable to me, he avoided lifting the mainboard and possibly causing some sort of damage he did not want to fix. My amp is '82 50 Watt, JCM 800 vertical inputs, 4010 combo amp, with I believe the 2403 circuit. Not the diode clipping and transistor channel-changing version of 800, mine is the all tube signal path.

I wonder what that post is doing there..
What do you mean by testing the red resistor?
If you want to check its value you don't need to clip anything. Just turn off the power and measure it with a DMM. Its a resistor, not a cap.. just saying
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Re: Marshall JCM 800 blowing fuses

Post by JMPGuitars »

Bieworm wrote:
Sun 06/07/20 1:46 pm
What do you mean by testing the red resistor?
If you want to check its value you don't need to clip anything. Just turn off the power and measure it with a DMM. Its a resistor, not a cap.. just saying
You might be right, but since the resistor is in circuit, it could be in parallel with another resistor. That would give the incorrect value. Gotta check the schematic first and identify the part before you know how to test a component.
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Re: Marshall JCM 800 blowing fuses

Post by ZeroPointEnergy »

When I did the reading, I got the expected value, 220. It was 221 and 218, switching probe sides. The reading was consistent and steady. But I am not sure if there is another resistor under the board, hard to say. But so far, the reading seems correct as that's what's printed on the mainboard, right underneath it. 220k.
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Re: Marshall JCM 800 blowing fuses

Post by Daviedawg »

You say that the board is marked 220k. But you have measured 220. Did you mean 220k?
Looking at the colour code on the resistor most to the front of the picture with 220k marked on the board I assume that is the one. The colour bands red red is 22. But it is hard in the pic to see what colour the third ring which is the multiplier is. If it is brown the component is 220ohms. If it is yellow it is 220kohm which is what it should be.

Dd
Edit: gold is the tolerance
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Re: Marshall JCM 800 blowing fuses

Post by crgfrench »

3rd band looks yellow in both pics to me. 220k.
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Re: Marshall JCM 800 blowing fuses

Post by ZeroPointEnergy »

Daviedawg wrote:
Tue 06/09/20 2:11 am
You say that the board is marked 220k. But you have measured 220. Did you mean 220k?
Looking at the colour code on the resistor most to the front of the picture with 220k marked on the board I assume that is the one. The colour bands red red is 22. But it is hard in the pic to see what colour the third ring which is the multiplier is. If it is brown the component is 220ohms. If it is yellow it is 220kohm which is what it should be.

Dd
Edit: gold is the tolerance
Consensus so far seems to be that the white/yellow strip should be yellow, and looks a bit yellow in one photo, but to my eye and in most photo's, it's more of an off white.

My meter is auto range, and just says 221ish or 218ish, depending on reversing the leads or not. So I do not really know for sure what the 220 means, but I assume it means 220K as that's what is called for, and that is mostly what the stripes indicate, so at this point, most clues suggest, it's a 220K.

But I defer to more professional opinions as I am new to this, but not new to troubleshooting and such.

~~~

I took PC programming and also had a PC business, sales and repair and I taught myself networking, etc. so I'm sharp with technical issues, but not experienced with amp repair,, yet.

I figure on building my power tube probe kit, today or tomorrow. After that, I can get a better idea what's going on. The good news it two fold. Wow do the new tubes sound great, and I really like em. And I found a local amp tech, and he's fitting me in next month.

Fella's, yesterday, I loaded my '82, 50 watt, Marshall JCM 800, with mostly,,, Telefunkins! I kid you not. It was glorious. Mullards for the rest. I like Mullards, but wow, I like Telefunkin better. I hear JJ owns them.

~~~

Best set of tubes I ever had, and I had a single preamp tube, wow do I wish I had 2 or more!, for the tone spots, V1 and V2.

Amazing, deeper and broader tones, pick sensitivity, but the tone was hard to describe, rich and deep and sorta vintage sounding, high quality.

So much so, I didn't need to roll off my tone for my bridge pickup, which I faithfully have had rolled off to 3/4, for ever! I turned that tone knob all the way up, and it still did not get ice picky, not too much high highs! I could not believe it!!!

~~~

That shocked me, on my SG Standard with stock pups and pots. I can't stand lower quality tubes now. I am hit, I have been changed, my eyes have seen the glory, hehe,, and I can't go back.

Nicest my rig has ever sounded since I've owned it, and that's going back some 12 yrs or so.

I can't wait to get this amp working properly and stops blowing fuses.
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Re: Marshall JCM 800 blowing fuses

Post by JMPGuitars »

ZeroPointEnergy wrote:
Thu 06/11/20 6:00 am
My meter is auto range, and just says 221ish or 218ish, depending on reversing the leads or not. So I do not really know for sure what the 220 means, but I assume it means 220K as that's what is called for, and that is mostly what the stripes indicate, so at this point, most clues suggest, it's a 220K.
Auto ranging still tells you what the actual number is (unless the meter is trash and should be thrown out). Try reading other resistors and look for a K or M somewhere on the screen, or some kind of indicator for the range it's in.
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Re: Marshall JCM 800 blowing fuses

Post by ZeroPointEnergy »

JMPGuitars wrote:
Thu 06/11/20 7:32 am
ZeroPointEnergy wrote:
Thu 06/11/20 6:00 am
My meter is auto range, and just says 221ish or 218ish, depending on reversing the leads or not. So I do not really know for sure what the 220 means, but I assume it means 220K as that's what is called for, and that is mostly what the stripes indicate, so at this point, most clues suggest, it's a 220K.
Auto ranging still tells you what the actual number is (unless the meter is trash and should be thrown out). Try reading other resistors and look for a K or M somewhere on the screen, or some kind of indicator for the range it's in.
Good call, thanks. That's kinda what I would have assumed. So I gave it another look, and your right, it was small in the upper corner. 220K is the reading. 220.1 and 218.3 when I swapped the leads.
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Re: Marshall JCM 800 blowing fuses

Post by crgfrench »

ZeroPointEnergy wrote:
Thu 06/11/20 6:00 am
I like Telefunkin better. I hear JJ owns them.
I'm pretty sure JJ owns Tesla.
Daimler sold Telefunken to Tech Sym Corp. in 1995. The current tube production unit seems to have been purchased by a US company in 2000, called "Telefunken USA" which is based in Connecticut and was renamed "Telefunken Elektroakustik" in 2009. They seem to only be focused on microphone reproductions. The tubes, if they are still manufactured, seem to be made in Germany if they are still making them -- they seem to have ceased production of tubes in either 2005 or 2013. Perhaps JJ bought them out of the insolvency of Telefunken?
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Re: Marshall JCM 800 blowing fuses

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crgfrench wrote:
Thu 06/11/20 1:03 pm
ZeroPointEnergy wrote:
Thu 06/11/20 6:00 am
I like Telefunkin better. I hear JJ owns them.
I'm pretty sure JJ owns Tesla.
Daimler sold Telefunken to Tech Sym Corp. in 1995. The current tube production unit seems to have been purchased by a US company in 2000, called "Telefunken USA" which is based in Connecticut and was renamed "Telefunken Elektroakustik" in 2009. They seem to only be focused on microphone reproductions. The tubes, if they are still manufactured, seem to be made in Germany if they are still making them -- they seem to have ceased production of tubes in either 2005 or 2013. Perhaps JJ bought them out of the insolvency of Telefunken?
I see, these tubes may be recent limited quantities production and is no longer in production as they did not show quite the same options as they do with standard production tubes. These tubes arrived with foam on the inside, even with the preamp tube. Super protected, and they sound fabulous.

I felt lucky having them. Thanks much for the ownership background. I was not sure about it, as the comment was handed to me, kinda as a passing comment. Maybe you also infer they were concentrating on microphone application for tubes as well. I had to guess if it was well suited for guitar amps, because the description was kinda vague if it was good for guitarists and mic preamps alike, but I bought it anyway. So glad I did. Made the best upgrades to the tone.
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